Catholic leaders in Australia are so concerned about a cache of abuse data to be released on Monday they have warned believers ahead of the shock awaiting them.

The Archbishop of Brisbane, Mark Coleridge, emailed a video message to churchgoers ahead of impending testimony that will highlight the extent of sex abuse in the Australian Catholic Church.

He warned the Royal Commission into how different institutions handled child sex exploitation had information that offered a "horrific portrait of appalling abuse", according to ABC.

In the video also sent to parents of children at Catholic schools, he said: "The Royal Commission is about to hold its final hearing into the Catholic Church which will be a very challenging time."

Archbishop Coleridge admitted there would be "grim moments and some shocks" in a message that will be aired in more than 200 churches around Brisbane this weekend.

"We have to shift the culture and that's a far more difficult thing to do," he added.

Coleridge's message comes after one of Australia's most senior Catholics and president of the Australian Catholic Bishops Conference, Archbishop Denis Hart, wrote a letter to 42 dioceses that will be read across the country this weekend.

He said: "For the victims and survivors, for the Catholic community and for many in the wider Australian community, this hearing may be a difficult and even distressing time...deeply mindful of the hurt and pain caused by abuse, I once again offer my apology on behalf of the Catholic Church."